(1) Field of the Invention
This invention relates to an agent for rendering harmless a halogen-containing gas such as a halogen-containing exhaust gas from, for example, an etching step in the process of producing a semiconductor device or a liquid crystal device, a CVD (chemical vapor deposition) process, and a cleaning process; and a method of rendering a halogen-containing gas harmless using the agent.
(2) Description of the Related Art
As a method of rendering harmless a halogen-containing gas such as a halogen-containing exhaust gas from, for example, an etching step in the process of producing a semiconductor device or a liquid crystal, a CVD (chemical vapor deposition) process, and a cleaning process, a thermal decomposition method, a wet method and a dry method are well-known.
The thermal decomposition method involves thermal decomposition of the above-mentioned exhaust gas by heating or burning the gas. The thermal decomposition method has problems such that a high temperature heating or burning is difficult to adopt in the case where a combustible gas is used, for example, in the process of producing semiconductor devices, and, in the case where the thermally decomposed gas is treated with water, a problem of waste water treatment is caused.
The wet method involves absorption of the exhaust gas with water or an aqueous alkali solution such as an aqueous sodium hydroxide solution. In the case when an aqueous alkali solution is used for absorbing the exhaust gas, problems arise in that semiconductor wafers are liable to be contaminated with an alkali, or a solid matter produced by the reaction of the aqueous alkali solution with a halogen-containing gas tends to clog the exhaust line of a treating apparatus. In the case when water is used for absorbing the exhaust gas, washing water is usually reused to reduce the amount of waste water, and consequently, the efficiency of the exhaust gas with water is rather low.
The dry method involves a treatment of the exhaust gas with a solid agent for rendering the exhaust gas harmless. The drying method is simple, and the problems encountered in the thermal decomposition method and the wet methods can be solved, and therefore, the dry method is widely used. As the agent for rendering the halogen-containing exhaust gas harmless, an agent comprising a salient amount of activated carbon has been heretofore widely used. However, there is a possibility that activated carbon violently absorbs and/or reacts with halogen-containing gases, which is accompanied, although rarely, by ignition or explosion due to exothermic reaction.
To resolve the problem encountered by activated carbon, zeolite which is incombustible has been proposed as an agent for rendering the halogen-containing exhaust gas harmless. Zeolite includes various types having different crystalline structures, and various compositions and pore diameters. Many proposals have been made as to the agent comprising zeolite and a method of rendering a halogen-containing compound harmless using the zeolite-comprising agent.
For example, Japanese Unexamined Patent Publication (hereinafter referred to as “JP-A”) No. S62-289222 discloses a method of rendering a halogen-containing gas harmless by using an agent comprising soda lime, combined with alumina and/or zeolite. JP-A H06-47233 discloses a method of rendering a halogen-containing exhaust gas harmless by bringing the halogen-containing gas into contact with zeolite having an average pore diameter of at least 9 Å. JP-A 2001-338910 discloses a method of rendering a halogen-containing gas harmless, which comprises contacting a halogen-containing gas with an agent comprising iron oxide, an alkaline earth metal compound and activated carbon, followed by contacting with zeolite such as X-type zeolite or faujasite-type zeolite. JP-A 2004-181300 discloses a method of treating an oxidative gas or an acid gas with an agent comprising an alkaline earth metal compound, an alkali metal compound, zeolite and a carbonaceous material. As the zeolite, MS-5A (CaA-type zeolite) and MS-13X (NaX-type zeolite) are mentioned. WO2003/033115 discloses a method of removing a halogen-containing gas by contacting the halogen-containing gas with granules comprising a solid base, a carbonaceous material, and a porous material comprised of an inorganic oxide. The inorganic oxide includes silica gel and/or zeolite such as A-type zeolite.
Thus, MS-5A (CaA-type zeolite) and MS-13X (NaX-type zeolite) are known as active ingredients for rendering a halogen-containing gas harmless. However, their capacities of rendering a halogen-containing gas harmless are not sufficiently desired.
As another zeolite, X-type zeolite comprising Na-LSX zeolite and a binder has been proposed as a gas-purifying molecular sieve absorbent in JP-A 2002-519188. However, this patent document specifically discloses on purification by adsorptive removal of only carbon dioxide gas.